I haven't seen this hint elsewhere; there is something similar in Mail, but not exactly. If you type in someone's initials in Spotlight, it will return their contact info.

[robg adds: This works, but it's not necessarily the most efficient way to find a contact (I prefer using Butler to do so by typing part of their name). In my case, there are 125,649 matches for 'rg' on my machine, only one of which is my contact card ... and it took quite a few seconds before the Address Book entry showed up in the results list.]

Running 10.4.2 on a Powerbook G4, typing in initials only shows me those address cards where contacts have the common "first-initial, last name email " ie. jsmith@internet.com . If the two letters do not appear side by side somewhere in the card, Spotlight does not find them. Searching for my initals, for example, does not bring up my contact info, but does find address cards I've made for applications I own, which have been serialized with my initials. Maybe my spotlight need to re-index?

Like robg, I've found the quickest way to an address isn't Spotlight, but for me, Quicksilver, a competitor to Butler. One key to bring up Quicksilver, type the name (first, last, or partial), and I see a list of matches.